1. Field of the Invention
The present invention relates, in general, to spacer grids used for supporting a plurality of fuel rods in a nuclear fuel assembly and, more particularly, to a spacer grid provided with a hybrid flow-mixing vane at the top of each intersection of inner straps, thus improving the efficiency of heat removal from nuclear fuel rods through flow mixing in coolant channels.
2. Discussion of the Related Art
As shown in FIG. 1, a contemporary fuel assembly for nuclear reactors has a square cross-section and includes a plurality of fuel rods. The fuel rods are regularly arranged by several spacer grids positioned along the axial direction.
The spacer grids for the above nuclear fuel assembly are normally fabricated by intersecting at right angles a plurality of thin straps, each having a height of about 4 cm, at their vertical slits to form a plurality of four-walled cells for receiving the fuel rods. In each strap of the spacer grid, the vertical slits are formed from the upper or lower edge of the strap and are regularly spaced at an interval equal to a desired pitch of the fuel rods. Each strap of a spacer grid is made of a metal strap and is divided into a plurality of unit straps on the basis of vertical slits. Grid springs and dimples are formed at each strap and are used for supporting a corresponding fuel rods.
The primary function of the spacer grid is to support a plurality of fuel rods in a nuclear fuel assembly while maintaining a constant gap clearance among fuel rods so as to form a plurality of coolant paths. Normally, a confined volume enclosed by four fuel rods is defined as a flow channel. The channels are connected to each other through rod gaps so that the coolant can pass freely between channels. Upon reactor operation, the fuel rods generate tremendous energy by the nuclear reaction, and the coolant flowing through the channels among the fuel rods is to remove the energy.
Typically, the fuel rods emit different thermal outputs due to a non-uniform neutron flux distribution in the reactor core, and accordingly, the coolant temperatures in the channels are also non-uniformly distributed. Under these conditions, if operational transients or unexpected conditions occur, such as a pump unexpectedly stopping to reduce the coolant flow in the reactor core or a power control rod being ejected from the core to exceed the nominal thermal outputs of other fuel rods around it, a thermal critical condition (called “dryout”) may occur on the surface of the hottest fuel rod cladding.
Dryout is a sudden increase in wall temperature, approaching a melting point, which results from a paralysis of heat transfer due to the generated vapor bubbles covering the heated surface and blocking of coolant supply adjacent to the surface. In such a case, the local overheating area in the fuel rod cladding is fused, allowing the radioactive materials restrained inside to leak into the core and consequently contaminating the nuclear reactor.
To minimize the risk of such incidents in the reactor and to attain a sufficient operational margin, an additional role of the spacer grid, one of flow mixing in the fuel assembly, has recently been emphasized. Flow mixing breaks down the thermal boundary layers formed around the fuel-cladding surface, to relieve the sharp temperature gradient near the heated surface. Flow mixing also reduces the imbalance of coolant temperatures among neighboring channels. These effects can assist in lower the temperature of the hottest channel and thus easily avoid a critical condition.
The spacer grid with a flow-mixing device produces a complex flow pattern when the coolant passes through the device. The flow pattern can be characterized, based on the flow-mixing pattern, as one of three types. First, violent eddies and turbulence are generated just downstream from the spacer grid, which are always present where the fluid flow encounters a blunt obstacle, but the pattern decays very quickly. Second, lateral or cross flow occurs due to a flow deflection toward neighboring channels, and is also limited to within a short distance from a flow deflector, the shape of which determines flow magnitude and direction. Third, swirl motion occurs within channels but becomes attenuated farther along the flow. Experiments show, however, that this flow pattern is sustained downstream from the spacer grid, farther than either turbulence or cross flow.
On the other hand, adopting the flow-mixing device in the space grid inevitably brings an increase of pressure drop. It yields a higher hydraulic load on the fuel assembly. In addition, excessive or sudden blocking by the mixing device may cause violent and unsteady eddies and in turn fuel rod vibration. In such a case, a continuous contact slip between fuel rods and the spacer grid may generate wear problems. Therefore, a desirable flow-mixing device should effectively produce the above three kinds of flow patterns, while reducing flow resistance and the likelihood of rod vibration.
A variety of flow-mixing devices for the nuclear fuel spacer grids have been proposed. Typical examples include U.S. Pat. No. 4,692,302 (inventors: Edmund E. Demario, et al., assignee: Westinghouse Electric Corp.), U.S. Pat. No. 5,299,245 (Inventors: Michael E. Aldrich, et al., assignee: B&W Fuel Company), and U.S. Pat. No. 6,236,702 B1 (inventors: Taehyun Chun, et al., assignee: Korea Atomic Energy Research Institute).
In U.S. Pat. No. '302 two mixing vanes are formed at the top of each intersection of the inner straps and are bent in opposite directions. This device generates a strong two directional lateral flow at the rod gaps, but a relatively weak swirl flow in the channel.
In U.S. Pat. No. '245, four mixing vanes are formed on the top of two intersected inner straps at each intersection. The flow-mixing vanes deflect the axial flow into four directional lateral flows. This device generates weak lateral flow at the rod gap, due to collision with the lateral flow from neighboring channels, and a moderate swirl flow in the channel.
In U.S. Pat. No. '702, four mixing vanes are formed at both sides of a triangular extrusion extending from the top of each intersection of the inner straps. These swirl vanes twist the main axial flow to rotate within a channel. Thus, a strong swirl flow is generated in the channel, but a weak lateral flow is generated at the rod gaps.
Enormous effort has been devoted to the flow-mixing devices of the spacer grid. Though there have been achievements, such as those of the flow-mixing devices mentioned above, there is still a strong need for further improvement in the thermal performance of nuclear fuel in order to attain a sufficient operating margin to guard against dryout.